This invention relates generally to apparatus for transferring a printed, written, or pictorial image from an original subject sheet to a master stencil sheet for use in making multiple copies of the image in a printing or duplicating machine. More particularly, it relates to apparatus sometimes referred to as an electronic stencil cutter, although it will be apparent as the description proceeds that certain novel features of the invention may usefully be employed in printing and duplicating apparatus other than for the stencil process.
The simplest and most widely used apparatus of this kind has a rotatable drum with means for mounting the original subject sheet and the stencil sheet side by side. A scanning head and a cutting head are supported on a carriage alongside the original subject sheet and the stencil sheet respectively. The scanning head includes a light source, an optical pickup focused on the subject sheet, and a photo-sensitive element which senses variations in light intensity of the image on the subject sheet. The cutting head has a perforating stylus with a very fine wire electrode pressed against the stencil sheet which has a special electrically conductive layer therein. A high voltage electrical circuit generates sparks at the point of contact between the electrode and stencil sheet. These sparks are synchronized with variations in light intensity reflected from the original image and thereby reproduce that image as holes burned in the stencil sheet.
The stylus current which generates the spark is inversely proportional to the intensity of the light reflected from the pattern on the original. That is, a bright reflection from a white or light background area on an original subject sheet produces no spark between the wire electrode and the stencil sheet, and no hole or holes are made in the stencil sheet. Conversely, dark, printed areas or lines on the original sheet produce sparks at the end of the electrode and burn holes through the corresponding areas in the stencil sheet. Later, during printing or duplicating, ink is squeezed through these holes onto copy sheets to reproduce the original dark areas exactly.
During the stencil-cutting process the drum is rotated while relative movement lengthwise of the drum is effected between the drum and the carriage supporting the scanning and cutting heads. U.S. Pat. Nos. 2,705,259; 3,006,992; 3,396,294; 3,801,738; and 3,823,262 show the general state of the art of electronic stencil cutters. In some of those patents, the drum is rotated while the scanning or cutter heads are moved along the drums. In others, the drum is rotated and moved axially while the heads remain stationary. Due to the simultaneous rotation of the drum and axial advance of the scanning and cutting heads relative to the drum, the original sheet is scanned along a helix, and the stencil sheet is perforated along an identical helix for same-size reproduction. The speed of advance is adjustable in many machines so that the pitch of the helical scanning and cutting can be varied. A low speed of relative axial advance results in perforation lines being close together and provides a high quality, fine, detailed reproduction, excellent for copying art work or photographs where the time required can be justified. With a higher speed advance, the perforation lines are farther apart but produce excellent stencils for copying printed matter such as menus, letters, and memos, and it requires less time.
Previously known electronic stencil cutters have a serious limitation because they can make only same-size stencils. Most standard office duplicating machines will not copy a full 8 1/2" wide because they use spring steel hold-down guides or strippers along the side edges of the copy sheets as the latter are fed across the stencil drum. Examples are strips 375 (shown in FIGS. 2 and 3 of Springer U.S. Pat. No. 3,835,772). These strips reduce the usable printable width of an 8 1/2" wide copy sheet to about 8" or less. If the original image is wider, as for example on a so-called "bleed" page where it extends right out to the edge, the original must be photostatically reduced before the stencil is made from it for use in these previously known duplicating machines.
As stated, previously known electronic stencil cutters have mechanism for varying the speed of advance of the scanning and cutting heads along the drum, or vice versa. One such mechanism is a motor-driven screw, threadedly engaging the movable part, the speed of advance being varied by changing the rotational rate of the screw through change-gears or clutches. Another such mechanism advances the movable part by means of cams or rollers bearing against the outer surface of a smooth rod, and the speed of advance is varied by changing the angle of contact against the rod; one such arrangement being shown in U.S. Pat. No. 3,396,294.
These are complex, costly, and imprecise, particularly in view of the special requirement for this apparatus that the motor drive means be manually overrideable, for example, when the operator moves the scanning head manually to the left hand edge of the image on the original subject sheet at the start of a stencil-cutting procedure. Another problem in this known apparatus is that the stylus assembly in the cutting head vibrates when it strikes the leading edge of the stylus sheet, once each rotation. Although the stylus is an expendable item, replaced frequently as the wire electrode burns short, conventional styluses are relatively costly because they comprise a multi-layer metal-and-plastic lamination, and each is made with a special vibration-dampening element which is thrown away with the stylus when it is replaced.
Still another problem in known electronic stencil cutter apparatus is the mechanics of clamping the original subject sheet and the stencil sheet onto the drum. In conventional apparatus, these sheets are held by complicated, cumbersome, and costly arrangements of levers and clamps.